This invention relates to an apparatus for controlling the rotational speed of a prime mover of a construction machine such as a hydraulic shovel or a crane.
A prime mover speed controller of this kind, such as the one disclosed in Japanese Utility Model Laid-Open No. 61-145849, is known which is capable of remote-controlling an engine governor. This controller transmits a value which represents the operation of an operational section to an operational value detector through a link or the like to obtain a command signal corresponding to the operational value. This command signal is transmitted to a control circuit which controls a motor which rotates to drive the governor.
Since this type of apparatus necessitates the provision of an operational value detector (e.g., potentiometer or pulse encoder) for detecting the value representing the operation of the operational section, it entails the following problems.
1. It is necessary to correct the output from the operational value detector by the control circuit because the linearity of the output from the operational value detector with respect to the operational value is not sufficiently high. PA1 2. A component such as a link for connecting the operating section and the operational value detector is needed. The number of component parts is thereby increased and the resulting structure is complicated. PA1 3. A process of adjusting the operating section and the operational value detector is needed. PA1 4. If a potentiometer is used as the operational value detector, a constant voltage power source for driving the potentiometer is needed. Since the output from the potentiometer is an analogue signal, it is necessary to cope with the problem of noise. The potentiometer has a specific structure which includes slide members and is therefore disadvantageous in terms of reliability and durability. Moreover, the provision of an A/D converter for converting the analogue signal from the potentiometer into a digital signal is necessitated if a control system in which the target engine speed is calculated with a microcomputer and in which a control lever of the governor is driven by a pulse motor is adopted. PA1 5. Ordinarily, in construction machines, the position of the control lever of the governor is constantly maintained and the machine is operated by depending upon a fuel injection control function of the governor. There is a risk of the operational section being accidentally operated because large vibrations or impacts are caused during working. There is therefore a need for a mechanical lock means for constantly maintaining the position of the operational section. PA1 6. In a case where this type of prime mover speed controller is applied to a hydraulic power shovel, certain problems in terms of operability are encountered, as described below. Hydraulic shovel working is performed by operating a pair of working levers disposed on the left-hand and right-hand sides of a cockpit to effect operations of a boom, an arm and a bucket and revolution. If the above-described operational section is provided in a panel disposed in a side section of the cockpit separately from these working levers, the operator must stop working by releasing one hand from the working lever when he wishes to change the engine speed during working. Even if the operational section is provided in a grip portion of one of the working lever in the conventional system, it is necessary to effect a rotary operation of the operational section to a position corresponding to the absolute value of the engine speed. The working is therefore stopped substantially in order to change the engine speed. This procedure is disadvantageous in terms of operability. PA1 7. In the case of a type of prime mover controller which has a set speed command means which issues a command to shift the rotational speed of the prime mover to at least one set speed (e.g., a controller which has a power mode and an economy mode and which shifts the engine speeds in accordance with the selected mode), the following problems are encountered. PA1 8. Each time the engine is started, it is necessary to operate the operational section to set a rotational speed suitable for starting. If in the conventional system the engine is controlled to rotate at the desired speed in synchronism with the starting operation, non-conformity of the operational position of the operational section with the engine speed takes place.
(a) There is a possibility of occurrence of non-conformity of the operational position of the operational section with the engine speed and, hence, a possibility of the operator being confused. PA2 (b) It is difficult to finely change the rotational speed on the basis of the set speed.